exitwhen: Fix compilation on FreeBSD

POSIX says WIFEXITED() and friends require <sys/wait.h>.  Linux
happens to leak those out other headers as well, but FreeBSD was
stricter.  Meanwhile, mingw lacks that header, so copy the paradigm
used elsewhere for conditional inclusion.

filters/exitwhen/exitwhen.c

@@ -1,5 +1,5 @@
 /* nbdkit
- * Copyright (C) 2019-2020 Red Hat Inc.
+ * Copyright (C) 2019-2023 Red Hat Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
@@ -44,6 +44,10 @@
 #include <errno.h>
 #include <sys/types.h>
 
+#ifdef HAVE_SYS_WAIT_H
+#include <sys/wait.h>
+#endif
+
 #include <pthread.h>
 
 #include <nbdkit-filter.h>